If you are an American Airlines (AA) Elite Gold or Platinum member, then you earn upgrades as you fly. For every 10,000 miles you fly, straight "butt in seat", you get four 500 mile upgrades. These upgrades can be used to upgrade into the next class of service from the ticket you purchased.
Just for reference, Executive Platinum level members do not earn 500 mile upgrades as they are upgraded on a complimentary basis.
Back to Gold and Platinum. The earning "year" for the 500 mile upgrades is March 1 to February 28/29. So every year, on March 1 you go back to ZERO and start earning all over again. Why this is important is that you may have been close to another 4 upgrades at February 28 this year; close to the next 10,000 increment. However, AA will just reset your upgrade mileage balance to zero and many people do nothing about this (I am even guilty about this in past years). However, last year I learned to call AAdvantage to get some upgrades based on where I was when the reset happened.
As an example. The Mrs had a balance of about 27,049 in her "miles towards upgrades" on February 28, 2013. On March 1, that balance went to 0. Had she reached 30,000, she would have gotten 4 upgrades. She called the Platinum desk and kindly requested some upgrades for almost reaching 30,000 and the customer service agent told her she would credit her 2 upgrades. The Mrs thanked the agent for the 2 and simply asked if it was possible to get 3 since she had gotten so close. The CS agent agreed and said 3 upgrades would be credited in 48 or so hours.
So, I highly suggest calling AAdvantage to get credit for any upgrades you can. A majority of people won't know their balance at February 28 since they don't keep track of this separately like I do. In that case, still call and inquire what the balance was since the agents can look it up. Be nice and you will receive.
Do it sooner than later!

